In conventional automotive engine control, the extent that certain well-known engine and emission system performance goals can be achieved is largely determined by the capacity to control the engine air-fuel ratio. In general, many conventional vehicle powertrain controllers PCMs attempt to maintain the engine air-fuel ratio at the well known stoichiometric ratio (.lambda.=1). This ratio is generally found to yield satisfactory engine performance.
Engine control systems that are capable of controlling fuel, air, and recirculated exhaust gas EGR, attempt to maintain the air-fuel ratio at stoichiometry by coordinating control of the quantity of fuel, air, and EGR admitted into the engine, based on predetermined relationships between those control parameters calibrated for the specific engine application, and based on the present engine operating condition.
Such control may not account for manufacturing variations or for disturbances to the control system, for example the inevitable system performance changes due to aging. As such, it is common in the art of engine control to sense the performance of the air-fuel ratio control itself, for example using an oxygen sensor located in the exhaust path of the engine to observe, in a conventional manner, the actual engine air-fuel ratio. The observed (sensed) air-fuel ratio may then be fed back to the engine controller, which may trim (adjust) one of the three control parameters in order to compensate for the variations or disturbances.
In many such systems, fuel is a high resolution control parameter, making it an attractive candidate when precise air-fuel ratio control is desired. However, such systems may be "fuel-lead" systems in that the driver directly sets a fuel command which is directly related to engine torque, and only indirectly sets the air and EGR commands. As such, fuel command adjustments tend to be more perceptible to the driver in these systems. Such perceptibility is generally considered to be a disadvantage, as it disturbs the torque command--particularly in transient maneuvers.
Alternatively, these fuel-lead systems may trim the quantity of air admitted into the engine in engine air-fuel ratio control. Because air, unlike fuel, is not directly controlled by the driver in these systems, air trim is less perceptible to the driver. However, air trim does not provide the resolution available with fuel trim, and air trim can only be used in certain engine operating regions.
Further, these fuel-lead systems may trim the quantity of EGR admitted into the engine for air-fuel ratio control. Like air trim, EGR trim is less likely to be perceived by the driver in many of these systems. Further, when the quantity of EGR and the ratio of fuel to air in the engine rise or fall together, such as when EGR is trimmed for air-fuel ratio control, the desired air-fuel ratio correction may be achieved while limiting the creation of oxides of nitrogen (an undesirable combustion product) in the engine. Still further, the engine spark command is less sensitive to EGR trim than to fuel or air trim. However, EGR is not available or desirable in certain engine operating regions, such as in high engine load regions, or at idle. Further, EGR control does not have the resolution available with fuel control.
In the above-described systems, there are advantages and disadvantages associated with trimming fuel, air or EGR in order to control air-fuel ratio. What is needed is a system control strategy that controls engine air-fuel ratio using all three control parameters in a manner that retains the benefits of each and minimizes their weaknesses.